Add `{Entry,VacantEntry}::insert_entry` #416

Job	Run time
clippy	17s
nostd_build (1.63.0, thumbv6m-none-eabi)	26s
miri	4m 33s
Check MSRV and minimal-versions	38s
nostd_build (stable, thumbv6m-none-eabi)	24s
tests (1.63.0)	55s
tests (stable, arbitrary)	43s
tests (stable, quickcheck)	36s
tests (stable, rayon)	1m 13s
tests (stable, rustc-rayon)	1m 16s
tests (stable, serde)	42s
tests (stable, borsh)	35s
tests (stable, std)	33s
tests (beta)	38s
tests (nightly, test build benchmarks)	36s
Success	1s
	14m 6s

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